If a block and tackle arrangement were rigged as shown in figure "G" in the illustration, the amount of force "P" required to hold the 334 pound load stationary would be __________. See illustration GS-0110.
• Mechanical advantage of block and tackle (how many parts of line actually support the moving block) • Relationship: Load = Mechanical Advantage × Effort (P), so P = Load ÷ Mechanical Advantage (ignoring friction) • Difference between the standing part, the fall (free end), and the parts of line that directly support the load
• In figure G, how many rope segments are directly lifting (supporting) the lower block and load, not counting the free end you pull on? • Once you know that number, what is the ideal mechanical advantage of this tackle, and what effort P would you compute by dividing 334 lbs by that number? • Which answer choice is closest to the force you calculate, assuming an ideal (frictionless) system as typically used in exam questions unless losses are stated?
• Be sure you only count the rope segments attached to the moving block when finding mechanical advantage, not the free end you hold • Confirm whether the standing part of the line is made fast to the fixed block or the moving block, because that changes the mechanical advantage • After calculating P, compare your result to all choices and pick the one that best matches your computed value
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